Comets and asteroids are both made up of rock, dust and debris. They also both travel around space in an orbit.
Asteroids are made up mostly of rock, which makes them denser than comets. They vary in size and range from diameters of a few feet to those that are miles in diameter. They are usually found between Mars and Jupiter and have stable, circular orbits.
Comets are usually located further out into space - usually beyond Neptune. This is because of their orbits, which are long and looping. These elliptical orbits keep them in outer space for long periods of time which means they are not often visible on Earth to the naked eye.
Comets are made up of rock and ice as well as chemicals like carbon dioxide, ammonia and methane. As they get closer to the sun they heat up and some of these gases, along with dust and other debris, forms tails behind them. The sun's rays make these tails glow.
Scientists regard comets and asteroids as being a description for a wide range of objects found in space, rather than as two distinct groups of objects. For example, ice has been found on some asteroids, while tails have been observed on others, although they are not created in the same way as comets.